Bering Land Bridge Could Have Been Inhabited for 10,000 Years

This map shows the outlines of modern Siberia (left) and Alaska (right) with dashed lines. The broader area in darker green (now covered by ocean) represents the Bering land bridge near the end of the last glacial maximum, a period that lasted from 28,000 to 18,000 years ago when sea levels were low and ice sheets extended south into what is now the northern part of the lower 48 states. (Photo : Wlliam Manley, Institute of Arctic and Alpine Research, University of Colorado)

North America's first inhabitants migrated from mainland Asia across a land bridge spanning the Bering Sea before traveling south and eventually settling across the land. But a group of anthropologists suggest that this migration was not rapid, and that these people settled upon the land bridge known as Beringia for as many as 10,000 years before moving on.

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Dennis O'Rourke, an anthropologist from the University of Utah contends that Beringia - which is now covered by the Bering Sea - was inhabited roughly 25,000 years ago until the earliest North Americans arrived upon the melting of glacial ice sheets some 15,000 years ago.

During what is known as the last glacial maximum - the last Ice Age - large ice sheets extended south into the Pacific Northwest, Wyoming, Wisconsin and Ohio, O'Rourke said. Large expanses of Siberia and Beringia were cold, but lacked glaciers, he said, which supports the idea that the area was inhabited.

There is, however, no concrete evidence that this habitation of Beringia occurred, as the whole region is now underwater. As such, there has been little credence given to the idea of Beringian settlement, but O'Rourke is trying to reignite that conversation.

In a column in the journal Science, O'Rourke and his colleagues offer a number of research suggestions that could lead to proving this theory.

This is corroborated, in part, by evidence of pollen, plant and insect fossils found in core samples drilled from the Bering Sea and Alaskan bogs. These samples offer evidence that the Bering land bridge contained biodiversity, including trees, shrubs and grasses.

"We're putting it together with the archaeology and genetics that speak to American origins and saying, look, there was an environment with trees and shrubs that was very different than the open, grassy steppe. It was an area where people could have had resources, lived and persisted through the last glacial maximum in Beringia," O'Rourke said in a statement. "That may have been critical for the people to subsist because they would have had wood for construction and for fires. Otherwise, they would have had to use bone, which is difficult to burn."

Despite the somewhat diminutive stature that the term "land bridge" implies, Beringia was expansive, measuring as many as 1,000 miles from north to south. O'Rourke contends that the prolonged settlement of Beringia helps explain why the Native American genome became separate from its Asian ancestor.

"At some point, the genetic blueprint that defines Native American populations had to become distinct from that Asian ancestry," O'Rourke said. "The only way to do that was for the population to be isolated. Most of us don't believe that isolation took place in Siberia because we don't see a place where a population could be sufficiently isolated. It would always have been in contact with other Asian groups on its periphery."

"But if there were these shrub-tundra refugia in central Beringia, that provided a place where isolation could occur" due to distance from Siberia, he said.

O'Rourke pointed to a study of the mitochondrial DNA of Native Americans samples throughout the Americas. The study found that the unique Native American genome arose more than 25,000 years ago, but didn't spread through the Americas until about 15,000 years ago.

"This result indicated that a substantial population existed somewhere, in isolation from the rest of Asia, while its genome differentiated from the parental Asian genome," O'Rourke said. "The researchers suggested Beringia as the location for this isolated population, and suggested it existed there for several thousand years before members of the population migrated southward into the rest of North and, ultimately, South America as retreating glaciers provided routes for southern migration."

"Several other genetic-genomic analyses of Native American populations have resulted in similar conclusions," he added.

Such evidence gives credence to the theory, but it cannot be proved unless archaeological evidence of human settlement in Beringia can be found, O'Rourke said.